Alkaloid syntheses carbon nucleophile synthesis

The iminium salts of 2,3-dihydropyridines are far more stable than the free bases and have been used extensively in the synthesis of alkaloids. N-Benzyl iminium salt 26, formed from the Polonovski-Potier reaction of V-oxide 25, was transformed into enol ether 27, which is a synthon for the unstable AT-benzvl-l, 2-dihvdropyridine 28 (Scheme 5) <2004LOC168>. The same transformation on a similar iminium salt has been used in the formation of macrocyclic marine alkaloids <1995TL2059>. Carbon nucleophiles, such as the silylenol ethers of esters, have been shown to undergo 1,2-addition rather than 1,4-addition to 2,3-dihydropyridinium salts <1999T14995>. 

Azirines (three-membered cyclic imines) are related to aziridines by a single redox step, and these reagents can therefore function as precursors to aziridines by way of addition reactions. The addition of carbon nucleophiles has been known for some time [52], but has recently undergone a renaissance, attracting the interest of several research groups. The cyclization of 2-(0-tosyl)oximino carbonyl compounds - the Neber reaction [53] - is the oldest known azirine synthesis, and asymmetric variants have been reported. Zwanenburg et ah, for example, prepared nonracemic chiral azirines from oximes of 3-ketoesters, using cinchona alkaloids as catalysts (Scheme 4.37) [54]. 

If the AM 1 -hydroxyalkyl)amide is not stable enough for isolation it is still possible to perform the amidoalkylation in a one-pot reaction. Thus the amide and the carbonyl compound (or the oxoamide) are treated with an acid catalyst in the presence of the carbon nucleophile, so that the equilibrium amount of the (hydroxyalkyl)amide is converted in situ into the /V-acyliminium ion, which is subsequently attacked by the nucleophile. This principle is often applied in the total synthesis of alkaloids -8. 

L-metilionine to -adenosylmethionine. In this process a positively charged sulphur is produced and facilitates the nucleophilic reaction. By the activity of diamine oxidase, the A -methyl-A -pyrrolinium cation is formed and after that the first alkaloid, hygrine. From hygrine, by way of acetyl CoA, hydrolysis and intramolecular Mannich reactions, other pyrrolidine and tropane alkaloids are synthesized cuscohygrine, hyoscyamine or tropinone, tropine and cocaine. The Mannich reaction involves the combination of an amine, an aldehyde or a ketone with a nucleophilic carbon. This reaction is typical in alkaloid synthesis, and can be written as follows ... 

J. H. Rigby, Vinyl Isocyanates as Useful Building Blocks for Alkaloid Synthesis, Synlett 2000, 1-12. W. D. Rudorf, Reactions of Carbon Disulfide with C-Nucleophiles, Sulfur Rep. 1991, 11, 51-141. 

A number of recent reviews exist about intermolecular and intramolecular reactions of the iV-acyl-iminium intermediate. Moreover, detailed accounts of the application in alkaloid synthesis have recently appeared. This chapter deals with reactions of species (1) with nucleophilic alkenes (and alkynes). Other synthetically useful nucleophiles like aromatic rings, active methylene compounds and organome-tallics will not be discussed here. In (1) R, and R are hydrogen or carbon substituents, and R may also be a hetero substituent, such as alkylamino or alkoxy. This chapter differs from previous reviews, as the material is ordered here on the basis of the structural features of the A -acyliminium intermediate. Major emphasis is placed on recent developments and stereochemical details. 

CONTENTS Nucleophilic Addition to the a-Carbon of H c-lams, P. R. Guzzo and M. J. MHler. Chemical Synthesis of Porphobilinogen and Studies of Its Biosynthesis. R Neier. Synthesis and Cycloaddiiion Reactiorts of Iso-Cortdensed Heteroaromatic F rroles. C. K. Sha. Azacyclopentadienyl Metal Compounds Historical Background and Recent Advances. C. Janiak and N. Kuhn. Recent Developments in the Synthesis of Marine Pyridoacridine Alkaloids. A. M. Echavar-ren. Alkaloid Synthesis Using 1 -Acylpyridinium Salts as Intermediates, 0. L. ConyinsandS. P. Joseph. Index. 

Preliminary mechanistic studies show no polymerization of the unsaturated aldehydes under Cinchona alkaloid catalysis, thereby indicating that the chiral tertiary amine catalyst does not act as a nucleophilic promoter, similar to Baylis-Hilhnan type reactions (Scheme 1). Rather, the quinuclidine nitrogen acts in a Brpnsted basic deprotonation-activation of various cychc and acyclic 1,3-dicarbonyl donors. The conjugate addition of the 1,3-dicarbonyl donors to a,(3-unsaturated aldehydes generated substrates with aU-carbon quaternary centers in excellent yields and stereoselectivities (Scheme 2) Utility of these aU-carbon quaternary adducts was demonstrated in the seven-step synthesis of (H-)-tanikolide 14, an antifungal metabolite. 

The addition of acid to Az-piperideine results in an iminium ion that readily reacts with nucleophilic species. This reaction has been particularly useful for the formation of carbon-carbon bonds in alkaloid total synthesis. For example, key steps in the total synthesis of ( )-porantherine (equation 36) (74JA6517), coccinelidine (equation 37) (77H(7)685) and eburnamonine (equation 38) (65JA1580) were acid-catalyzed ring closures between A2-piperideine derivatives and ends. Even the weakly nucleophilic carbon-carbon double bond can participate in this type of reaction (80JA5955), as has been demonstrated by a recent total synthesis of a morphinan derivative (Scheme 13). 

An impressive number of alkaloids has been generated from the synthon (202), which is accessible by an acid catalyzed rearrangement of the appropriately substituted cyclopropyl-imine. The endocyclic enamine (202) should react with electrophiles on the /8-carbon in a process which simultaneously renders the a-carbon electrophilic and therefore susceptible to capture by nucleophilic reagents. The application of this methodology to the synthesis of ipalbidine (191a) and septicine (204) is shown in Scheme 30. Here, the unusual 3-phenylthio-2-pyrroline intermediate (203) serves as a relatively stable equivalent synthon of the unsubstituted 2-pyrroline analogue which is notoriously unstable (77ACR193). 

Azirines can be synthesised, enantioselectively if required using a natural alkaloid as base, from the O-tosyl derivatives of the oximes of 1,3-keto esters in this synthesis the carbon is the nucleophilic centre and it is the nitrogen which is attacked with departure of tosylate. ... 

Villalstonine (109) has been partially synthesized (Scheme 32) from its two halves , pleiocarpamine (110) and macroline, in an analogous synthesis, a process which in this case reverses a degradation of the alkaloid. It is interesting to note that the partial synthesis may well procieed via 11), in which electrophilic attack has occurred at the -position of the pleiocarpamine indole nucleus. As in the example discussed earlier (see p. 211) this / -substitution is secured by a subsequent intramolecular nucleophilic addition to the a-carbon, C-2. 

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